1. Field of the Invention
The present invention is related to a switching-mode power supply (SMPS) and the operation method thereof.
2. Description of the Prior Art
SMPS is utilized extensively in the current consumer electronic devices. SMPS controls the windings to be energized and de-energized through a power switch for providing an output power source of desired specification. For instance, when at light load or no load, SMPS may operate in constant voltage mode, providing a substantial constant voltage source independent to the output current; alternatively, when at heavy load, SMPS may operate in constant current mode, providing a roughly constant current source independent to the output voltage.
FIG. 1 is a diagram illustrating conventional SMPS 10 of a flyback topology, capable of operating in the constant voltage mode or the constant current mode using primary side control. Bridge rectifier 12 roughly rectifies alternating-current (AC) power source VAC to input power source VIN. Primary winding 24 of transformer 20, power switch 15, and current detection resistor 36 are coupled in series between input power source VIN and a ground terminal. When switch controller 18 turns on power switch 15, primary winding 24 energizes; when switch controller 18 turns off power switch 15, transformer 20 de-energizes through secondary winding 22 and auxiliary winding 25. Rectifier 16 and capacitor 13 roughly rectify the electrical energy released from secondary winding 22 and provide output power source VOUT to output load 38. Rectifier 28 and capacitor 34 roughly rectify the electrical energy released from auxiliary winding 25 and thus provide operation power source VCC to switch controller 18. Startup resistor 26 provides the current required to build up voltage of operation power source VCC during a startup period of time. Voltage dividing resistors 30 and 32 forward to switch controller 18 divided voltage by dividing the reflective voltage across auxiliary winding 25, which, when transformer 20 de-energizes, corresponds approximately to the voltage across secondary winding 22. By detecting the reflective voltage through pin FB, switch controller 18 is able to monitor the voltage across secondary winding 22 and accordingly controls power switch 15.
FIG. 2 is a diagram illustrating a partial circuitry of conventional switch controller 18a in constant voltage mode. When power switch 15 is turned off, sample/hold circuit 42 samples the voltage of pin FB and generates feedback voltage VFB. Error amplifier 44 compares feedback voltage VFB with reference voltage VREF1 to generate compensation voltage VCOM. Comparator 50 compares compensation voltage VCOM with detection voltage VCS on pin CS. Comparator 52 compares current limiting voltage VCS-LIMIT with detection voltage VCS. The outputs of comparators 50 and 52, as well as the clock output of oscillator 46, are all coupled to gate logic controller 48, which accordingly controls power switch 15 via pin GATE. In addition, if voltage of output power source VOUT is well regulated, feedback voltage VFB is virtually equal to reference voltage VREF1 due to the negative feedback loop.
Circuitries relative to constant current mode are not shown in FIG. 2, as various SMPS of constant current mode and the control method thereof are already disclosed in the known art, such as U.S. Pat. No. 7,016,204 “Close-loop PWM Controller for Primary-side Controlled Power Converters”, U.S. Pat. No. 7,388,764 “Primary Side Constant Output Current Controller”, U.S. Pat. No. 7,110,270 “Method and Apparatus for Maintaining a Constant Load Current with Line Voltage in a Switch Mode Power Supply”, and U.S. Pat. No. 7,505,287 “On-time Control for Constant Current Mode in a Flyback Power Supply” etc.